There is a constant need in the semiconductor industry to increase the number of dies that can be produced per silicon wafer. This need, in turn, encourages the formation of smaller die. Accordingly, it would be beneficial to be able to form smaller structures and devices on each die without losing performance. For example, as capacitors are designed to take an ever decreasing amount of die space, those skilled in the relevant art have sought new materials with which to maintain or even increase capacitance despite the smaller size.
One such material is tantalum pentoxide (Ta2O5), which can be used as the dielectric in the capacitor. Oftentimes, an electrically conductive layer, such as one made of hemispherical silicon grain (HSG), underlies the tantalum pentoxide and serves as the capacitor's bottom conductive plate. With other dielectrics, it is preferable to have a layer of polycrystalline silicon (polysilicon) deposited over the dielectric to serve as the capacitor's top conductive plate. If polysilicon is deposited directly onto tantalum pentoxide, however, several problems will occur. First, silicon may diffuse into the tantalum pentoxide, thus degrading it. Second, oxygen will migrate from the tantalum pentoxide, resulting in a capacitor that leaks charge too easily. Further, the oxygen migrates to the polysilicon, creating a layer of non-conductive oxide, which decreases the capacitance. This can also be a problem when using barium strontium titanate ((Ba, Sr)TiO3, or BST) as the dielectric.
In order to avoid these problems, it is known to deposit a top plate comprising two conductive layers. Polysilicon serves as the upper layer of the plate, with a non-polysilicon conductive material interfacing between the tantalum pentoxide and polysilicon. One such material often used is tungsten nitride (WNX, wherein X is a number greater than zero). However, other problems arise with this process. Specifically, by the end of the capacitor formation process, a layer of non-conductive oxide often forms between the two conductive layers of the top plate. For ease of explanation, this non-conductive oxide will be assumed to be silicon dioxide (SiO2), although other non-conductive oxides, either alone or in combination, may be present.
Without limiting the current invention, it is theorized that the tungsten nitride is exposed to an ambient containing oxygen. The tungsten nitride adsorbs this oxygen due to bonds located on the grain boundaries of the tungsten nitride surface. Once the polysilicon layer is deposited, the device is then exposed to a thermal process. For example, the capacitor may be blanketed with an insulator, such as borophosphosilicate glass (BPSG). The BPSG layer may not be planar, especially if it is used to fill a trench in which the capacitor is constructed. Heat is applied to the die to cause the BPSG to reflow and thereby planarize. The heat can cause the oxygen at the tungsten nitride surface to diffuse into the polysilicon, wherein the oxygen and silicon react to form silicon dioxide.
Regardless of the exact manner in which the silicon dioxide layer is formed, the result is that the HSG/Ta2O5/WNX/SiO2/polysilicon layers form a pair of capacitors coupled in series, wherein the HSG/Ta2O5/WNX layers serve as one capacitor and the WNX/SiO2/polysilicon layers serve as the second capacitor in the series. This pair of capacitors has less capacitance combined than the single HSG/Ta2O5/WNX/polysilicon capacitor that was intended to be formed.
Other problems can occur with the association of WNX and Ta2O5. For example, it is possible for the WNX to serve as the bottom plate of a capacitor, underlying the Ta2O5 dielectric. In that case, the deposition of the Ta2O5 or a subsequent reoxidation of that layer may cause the WNX layer to incorporate oxygen, thereby reducing capacitance.
It should be further noted that capacitor formation is not the only circumstance in which such problems can occur. There are many situations in which an in-process multi-layer conductive structure is exposed to oxygen and is subjected to conditions that encourage oxidation. Another example can be seen in the formation of metal lines. A layer of tungsten nitride, or perhaps tantalum nitride, may serve as an interface between the conductive material of a via and the metal line. If the interface is exposed to an ambient containing oxygen, then a thermal process involving the alloying or flowing of the metal in the metal line could cause a similar problem with oxidation, thereby hindering electrical contact.
As a result, there is a specific need in the art to prevent or at least decrease the degradation of capacitance in capacitors and of electrical communication in metal lines. There is also a more general need to prevent or at least protect against or minimize the migration of oxygen in relation to a conductive layer of a semiconductor device.